Current limiting circuit breakers are well known in the prior art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 3,943,316, 3,943,472, 3,943,473, 3,944,953, 3,946,346, 4,612,430, and 4,618,751 which are assigned to the same assignee as the present application, and which are hereby incorporated by reference. Basically, a current limiting circuit breaker comprises a base and cover, a stationary contact, a movable contact secured to a rotatable blade, arc interrupting chamber, an operating mechanism for opening and closing the contacts, and a trip unit which releases the operating mechanism when a predetermined amount of current is exceeded.
Before the present invention, molded case current limiting circuit breakers were large, labor intensive, part intensive devices that had several areas of performance imitations. These circuit breakers provide movable contact arrangements coupled to operating mechanisms that open the circuit at high level short circuits. This is accomplished through the use of thermally responsive tripping elements, magnetic tripping elements, and parallel conductor blow open designs respectively.
A need, therefore, exists for an improved circuit breaker design that requires fewer parts, is easier to assemble, and is compact in design.
Current limiting circuit breakers require a single low-mass blade design and thusly the resistance allocation of the circuit breaker is skewed toward the limiter. This places rigorous requirements on the trip unit thermal section in that it must respond quickly to protect the limiter from burnout and use only a relatively small percentage of the total circuit breaker resistance so that total circuit breaker resistance is minimized. Some prior art circuit breakers use current transformers to accomplish this task. This approach is more expensive, has more parts, and may not be suitable for direct current applications. Some prior art current limiting circuit breakers use a conventional bimetal (thermal) approach, however, its overall circuit breaker resistance is significantly higher.
Thermal-magnetic circuit breakers interrupt current flowing through a circuit that exceeds a predetermined value. Generally, the thermal portion, of the circuit breaker's trip unit, determines when an overload conditions exists and then "trips" the circuit breaker, while the magnetic portion causes the circuit breaker contacts to remain closed during a short period of time while a high current level is experienced, such as during initial start up of certain types of equipment (i.e. electric motors). This (short) initial current is commonly called inrush current. Different types of equipment require various amounts of inrush currents. Therefore it is desirous to be able to adjust the level at which the circuit breaker will trip, so that nuisance tripping will not occur during the start up of this equipment. The magnetic portion can be adjusted to trip the circuit breaker at a particularly high level of current, commonly called the magnetic trip level because the trip unit uses a magnetic flux circuit to determine the level of current flowing through the current path.
A method most commonly used to adjust the magnetic trip level is to adjust the magnetic trip force required to trip the circuit breaker. The current path is routed through the middle of a yoke having an armature proximate thereto. A spring/screw assembly is connected to the armature at one end and the tripping mechanism and the other end. As current flows through the current path, a magnetic flux current is generated in the yoke, creating a magnetic force that pulls the armature towards the yoke. The greater the current, the greater the magnetic force and the more the armature travels towards the yoke. At a predetermined current level, the armature has travelled far enough to trip the circuit breaker. The spring force in the spring/screw assembly serves to counteract the magnetic force. The predetermined current level is established by varying the spring force by changing the length of the spring/screw assembly. The length of the spring/screw assembly can be varied by threading the screw into and out of the spring. In the prior art the magnetic adjust screw engages all of the active coils of the spring, creating calibration errors among other things. The torque required to engage the spring increases dramatically with the number of coils engaged resulting in spring wind-up when a certain nominal limit of coils are engaged. In addition, since spring rate is a function of the number of active coils, as more coils are engaged, the spring rate of the spring increases creating errors in the accuracy of the high-low magnetic adjustment range of the trip unit.